The invention relates generally to wind turbine generators and more specifically to a method of interconnecting parallel power converters for the wind turbine generators to eliminate a common mode current, which circulates between the parallel power converters.
Generally, wind turbines use the wind to generate electricity. The wind turns multiple blades connected to a rotor. The spin of the blades caused by the wind spins a shaft of the rotor, which connects to a generator that generates electricity. Specifically, the rotor is mounted within a housing or nacelle, which is positioned on top of a truss or tubular tower, which may be as high as about 100 meters. Utility grade wind turbines (e.g., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox. The gearbox may be used to step up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is provided to a utility grid. Some turbines utilize generators that are directly coupled to the rotor without using a gearbox. Various types of generators may be used in these wind turbines.
Many devices, such as wind turbines, include power converter systems. A power converter system is typically used to convert an input voltage, which may be fixed frequency alternating current, variable frequency alternating current, or direct current, to a desired output frequency and voltage level. A converter system usually includes several power semiconductor switches such as insulated gate bipolar transistors (IGBTs), integrated gate commutated thyristors (IGCTs or GCTs), or metal-oxide semiconductor field effect transistors (MOSFETs) that are switched at certain frequencies to generate the desired converter output voltage and frequency. The converter output voltage is then provided to various loads. Loads as used herein are intended to broadly include motors, power grids, and resistive loads, for example.
FIG. 1 is a block diagram of a typical power system coupled to a wind turbine with synchronous wound-field or permanent magnet generator and implemented according to one aspect of the invention. The power system 10 is configured to provide AC output power to grid 21. A wind turbine 12 is configured for converting wind energy to mechanical energy. The wind turbine is coupled through a gear box 19 to generator 14 or alternatively coupled directly to generator 14. Wind energy is captured by the rotation of the wind turbine's blades, and generator 14 is configured by a power converter system 20 controlled by converter control system 24 for generating a variable frequency input power. The power is transformed to appropriate voltage by one or more transformers 22 and supplied to the power grid 21.
To accommodate the need for greater power from windfarms, individual wind turbine generators are increasingly being provided with higher power output capability. To accommodate the higher power output from the wind turbine generators, some wind turbine systems are provided with multiple parallel converters (also known as converter threads). Multiple parallel converters may also provide an advantage in wind converters due to the desire for high availability and low distortion
Typically, power converter systems use multiple power converter bridges in parallel with gating control to expand power-handling capability. In wind turbine applications, a power converter bridge usually refers to a three-phase converter circuit with six power switches. In order to meet both grid side and machine side power quality requirements, such systems generally use very large and costly filters to smooth out pulse width modulated waveforms. Such systems sometimes cause overheating of the generator and/or transformers and other distortion-sensitive equipment due to high harmonic components, when the large and costly filters are minimized.
FIG. 2 is a block diagram of a typical power system employing multiple parallel converters. Power system 10 is configured for supplying power to a load 21. A generator source 14 is configured to generate an AC input power. The AC input power is provided to power converter system 20. The power converter system 20 comprises converter 20-1 through 20-N. The converters are coupled in parallel and configured to receive the AC input power from the generator source 14. The power converter system 20 is configured to convert the AC input power to an AC output power. The AC output power is provided to load 21. Loads may include motors, power grids, and resistive loads, for example. Although grids are traditionally suppliers of power, in most wind turbine system embodiments, wind turbine power is supplied to a utility grid, which acts as a load.
The plurality of multiple parallel converters, each one of which (also called threads) has a fraction of the net system rating. These converter threads are tied together on both the input and output ends to form a net current/power rating on both the input and output that is directly related to the number of converter threads in parallel. Typically, one side of the converter is connected to a common power source (for example the grid) and the other to a plant (for example a generator). The circuit connecting the converter to the power grid will usually be referenced to ground. For cost and size reasons, each thread is connected to a common point on the grid and the plant with conductors that are sized in accordance with the rating of each thread and not the system rating.
Converter control system 24 is configured to provide control signals for the operation of the power converter system 20. The converter control system 24 is coupled to the power converter system 20 and is configured to drive the converter system according to predesignated switching patterns. The predesignated switching patterns provided by the converter control system 24 may provide for synchronous gating of the multiple parallel converters (20-1 to 20-n) or may provide an interleaved manner of control for each converter thread with phase displaced gating signals to reduce overall switching harmonic components due to cancellation of phase shifted switching waveforms.
FIG. 3 is a block diagram of a typical thread of a power converter system. Wind turbine embodiments, for example, typically comprise three-phase power converter systems. Converter 20-1 represents one thread of power converter system 20. Converter 20-1 comprises generator converter bridge 30 for AC-DC conversion, DC link 35, and load converter bridge 40 for DC-AC conversion at a suitable voltage and frequency. Generator converter bridge 30 may be implemented using six power semiconductor switches 45. Similarly, load side bridge 40 may be implemented using six power semiconductor switches 45. Generator side chokes 50 and load side chokes 55 may be sized to enable either non-interleaved or interleaved gating
Switching of power semiconductors in the converter threads causes a difference in voltage between the parallel converters, which creates a common mode current that flows between the converter threads, even without having a ground fault on the system. The common mode current will flow in a circular loop between the power converter threads, but not have any impact on the net current in either the grid or the plant. Common mode chokes 60 suppresses the high frequency (switching frequency range) common mode cross current that links both generator side converters and the load side converters.
FIG. 4 illustrates common mode current flow in a power system converter with n-paralleled converter threads (20-1 to 20-n) connected to a grid 21 and to a wind turbine generator 14. For example, it is possible that a current can flow into thread T1_L_Ia 110 and out T1_G_Ia 115 and return through thread T2_C_Ia 120 and T2_L_Ia 125. There are many combinations of loops for such current that will not affect the net current. However, these common mode currents, as well as normal mode circulating currents, force converter switching devices and other components to operate closer to thermal limits. Further, these common mode currents may cause a direct error in the measurement of ground fault currents of that loop, thereby making fault detection more difficult. Large common mode inductors are required to limit the amount of circulating common mode current between the converters, as well as, large normal mode reactors are required to limit circulating normal mode current where phase shifting is utilized to reduce net distortion.
Accordingly, there is a need to provide a structure and method for interconnecting the power converter in a manner so as to reduce or eliminate the common mode current that flows between the parallel converter threads, without the need for common mode inductors, coupled to a capability to phase shift multiple threads to reduce the need for bulky filters.